Abstract: The invention is directed to a method for producing an open phototrophic culture with improved storage compound production capability. In accordance with the invention, a starting culture is submitted to selective pressure, thus giving a competitive advantage to storage compound producing species, by subjecting said starting culture to a cycle of alternating dark phases and light phases and providing limitation of availability of essential growth nutrients in one or more of said light phases. The resulting culture can be used to provide storage compounds in improved yields.